In the fields of integrated optical devices and electro-optical devices, polarization mode converters are known. These converters receive light polarized in a first mode and provide light at an output in a second polarization mode.
Mode converters can be either active or passive. An example of an active mode converter is U.S. Pat. No. 5,566,257 to Jaeger et al, which is incorporated herein by reference. The device of the '257 patent makes use of the electro-optical effect to induce polarization mode conversion.
FIG. 1 illustrates a conventional semiconductor optical waveguide 50 using a compound semiconductor 52, such as gallium arsenide (GaAs). The semiconductor layer 52 has an optical ridge 54 through which the majority of the optical power is transmitted. Semiconductor layer 52 preferably sits atop a substrate 56. In ridge 54 there are two principal axes that provide transmission along axes similar to electric and magnetic field lines. These axes are parallel to the dominant electric field components of the TE-like and TM-like modes. These axes will be referred to here simply as TE and TM. The TE and TM axes can be aligned to geometric X and Y axes. Light transmitted into the waveguide can be thought of as being broken into constituent elements and transferred along the waveguide. At the output of the waveguide, the light from each of the principal axes is combined to form the output light.
Due to the anisotropic nature of many waveguides, transmission of the fundamental waveguide modes, polarized along the principal axes, may not occur at the same speed, due to a phenomenon known as modal birefringence, where each of the principal axes has a different effective index of refraction. The term principal axes of a waveguide, or principle waveguide axes, is used to describe the two directions that are parallel to the dominant electric field components of the two lowest order hybrid modes of the waveguide. The orientation of the principal waveguide axes is dependent on the waveguide's geometry and the principal optical axes of the material forming the waveguide.
FIG. 2 illustrates the waveguide 50 of FIG. 1, with a pair of electrodes 58a and 58b surrounding the ridge 54. When an electric field 60 is applied between electrodes 58a and 58b, the principal axes, U and V, rotate. Additionally, the electric field has an effect on the birefringence of the optical paths. The birefringence, as described above, is a measure of the differing speeds of light transmitted through the crystal structure of the waveguide. When the two axes have been rotated by 45° from the TE and TM aligned axes, the geometry and birefringence allow for “full mode conversion” to result.
FIG. 3a illustrates the rotated axes and an incident input light that is TM aligned. The TM aligned light 62 is resolved into two components, 62u and 62v, equal in magnitude, each of the two components 62u and 62v aligned to one of the principal axes. If birefringence was held to zero the incident light 62 would be resolved into equal U and V components, 62u and 62v, propagated along the length of the waveguide and at the output the components would recombine to provide an output that was TM aligned. However, if the propagation speed along the axes is different, the two resulting outputs will be out of phase with each other. If the phase difference is a multiple of 360°, the outputs will be out of phase but phase aligned, however if the U and V components are 180° out of phase, or an odd numbered multiple of 180°, the output, as shown in FIG. 3b, will be a combination of the u component and the negative of the v component, 62u′ and 62v′. The combination of these outputs results in an output light that is TE aligned. Thus, the combination of geometry and birefringence results in a phase shift that simulates mode conversion.
FIG. 4 illustrates the effect of an electrical field oriented in a <011> direction on the geometry of the principal axes of a <01 1> directed waveguide, fabricated on a (100) cut electro-optic crystal with 43 m symmetry. As the field is increased from zero, the axes 66 and 68 rotate from a TE and TM alignment by an angle of θ. As the field increases, θ approaches 45°. A similar effect occurs with the birefringence, in that the birefringence increases with the strength of the electric field across the ridge 54, resulting in a greater disparity in the velocities. One skilled in the art will appreciate that the existence of principal axes implies the existence of some initial modal birefringence in the TE and TM axes to force θ=0 at zero electric field.
FIGS. 5 and 6 illustrate a passive mode converter 70 known in the art, that relies upon a structural feature to provide the necessary rotation of the principal axes. FIG. 5 is a top view of the converter 70, which is fabricated from a crystal 72, typically a compound semiconductor, which has a central ridge 74. FIG. 6 illustrates a cross-sectional view along cut line A-A. The converter 70 is shown with crystal 72, having ridge 74, mounted on substrate 78. Atop ridge 74 are a series of periodic layers of crystal 76. A mode converter of this sort is described in “Polarization rotation in asymmetric period loaded rib waveguides” published in Appl. Phys. Lett 59(11), 9 Sep. 1991. The periodic layers 76 form a staggered pattern, alternating about a centre line in the waveguide ridge 74. The presence of these layers causes a slight perturbation in the alignment of the principal axes under the layer 76. Over the length of the mode converter 70 these perturbations combine to provide full mode conversion even though the principal axes are not in their ideal 45° orientation. The perturbation in the principal axes is caused by the loading of ridge 74 by layers 76. Birefringence is also enhanced by the selection of a different compound semiconductor for layer 76. On one described embodiment, the waveguide 72 is formed from InP, while the layers 76 are formed from a “1.3Q” crystal composition. The combination of the materials is specifically chosen to mitigate, and preferably to eliminate, any shear stress caused by a size differential in the crystal matrices. The perturbations allow for mode conversion in a limited bandwidth of light. The periodicity and pattern applied in periodic layer 76 are designed to specifically interact with selected wavelengths, and as a result the overall mode converter 70 does not operate over a wide bandwidth.
FIG. 7 illustrates further passive structures known to have rotated principal axes. FIG. 7 illustrates a mode converter 80 made of a crystal 82 and having a waveguide ridge 84 that has an outwardly sloping slide. The non-square structure of the waveguide ridge 84 results in polarization modes that are not perfectly aligned with the TE and TM axes. Though the orientation of these axes depends on the exact shape of the waveguide ridge 84, proper design of the waveguide ridge 84 can result in rotated principal axes. This allows for a mode converter to be designed using geometric asymmetry in the waveguide ridge 84, which permits the waveguide axes to deviate from the normal axes of the substrate.
Though the system illustrated in FIG. 2 does effectively modulate the polarization mode in phase with an electric signal (generating an electric field across electrodes 58a and 58b), the field strength must be sufficient to rotate the principal axes to near 45°. When acting as a simple mode converter, this system has the drawback of requiring excessive DC voltage, whereas other converters can be passive, and thus less prone to electrical failure and performance drift. The modulation in the prior art is obtained by applying a large DC bias voltage to get the axes orientation to near 45°, then a smaller AC voltage is used to switch between on-off states. In the course of getting the axes to near 45°, the modulator passes through multiple on-off states with progressively better mode conversion efficiency. The DC voltage used to bias the modulator can vary over time as the properties and characteristics of the modulator change due to temperature, stress and fatigue. As a result the modulator requires periodic recalibration, which typically requires that the modulator be taken off-line.
Systems such as those illustrated in FIGS. 5, 6 and 7, though offering passive mode conversion, do not provide polarization modulation, and require sophisticated manufacturing techniques to achieve a specific, repetitive pattern that induces sufficient birefringence and axes rotation. The mode converter of FIGS. 5 and 6 requires the deposition of a 1.3Q layer on the waveguide, and then an etching process to selectively remove portions of the 1.3Q layer to leave the required pattern. Other mode converters employ internal repetitive structures to effect a rotation of the principal axes, and as a result are equally difficult, if not more difficult, to produce. As noted above, passive mode converters of this design function only over a narrow optical bandwidth, and are inefficient when used in electro-optic modulation. The inefficiency arises from the fact that in each waveguide section, the principal axes are not aligned closely to 45° and as a result the electro-optic effect is not acting fully in the same direction as the principal axes. The mode converter of FIG. 7 provides some degree of axes rotation, and has a static birefringence that is useful for passive mode conversion, but does not provide a mechanism to modulate the polarization.
It is, therefore, desirable to provide an optical waveguide suitable for use as both a passive mode converter and an active polarization modulator.